Fix wording in the build script flock64 rule

Co-Authored-By: gnzlbg <gnzlbg@users.noreply.github.com>

Author: elichai

libc-test/build.rs

@@ -2230,7 +2230,7 @@ fn test_linux(target: &str) {
 
             t if t.ends_with("_t") => t.to_string(),
 
-            // This is either a struct or a typedef to a struct
+            // In MUSL `flock64` is a typedef to `flock`.
             "flock64" if musl => format!("struct {}", ty),
 
             // put `struct` in front of all structs:.